Neurospora crassa and the
One Gene - One Enzyme Theory

Neurospora crassa is an ascomycete, the red bread mold. Like all fungi, it reproduces by spores.

It produces two kinds of spores: Neurospora is particularly well suited for genetic studies because (If crossing over should occur, what spore patterns could be produced?)

The One Gene - One Enzyme Theory

Sucrose, a few salts, and one vitamin — biotin — provide the nutrients that Neurospora needs to synthesize all the macromolecules of its cells.

Geneticists George W. Beadle and E. L. Tatum

Beadle and Tatum reasoned that radiation had caused a gene that permits the synthesis of thiamine from the simple ingredients in minimal medium to mutate to an allele that does not.

The synthesis of thiamine from sucrose requires a number of chemical reactions, each one catalyzed by a specific enzyme.

By adding, one at a time, the different precursors of thiamine to the medium in which their mutant mold was placed, they were able to narrow down the defect to the absence of a single enzyme.

Thus, in this example, the conversion of precursor C to precursor D was blocked because of the absence of the needed enzyme (c).

This led them to postulate the one gene - one enzyme theory: each gene in an organism controls the production of a specific enzyme. It is these enzymes that catalyze the reactions that lead to the phenotype of the organism.

Today, we know that, in fact, not only enzymes, but all the other proteins from which the organism is built are encoded by genes.


Galagan, J. E., et al. report in the 24 April 2003 issue of Nature the completion of the sequencing of the entire genome of N. crassa. Its 38,639,769 base pairs of DNA encode:

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23 February 2011